Workers at three labs who came into contact with the samples were not wearing full protective gear because they thought the spores were safe.

Some of the anthrax may have become airborne in two of the labs, while live bacteria was found on material gathered for disposal.

In a statement, the CDC said the risk of infection among members of the public or family members of affected staff was low.

However, the agency confirmed 75 staff members were being monitored or receiving antibiotics as a precaution.

The CDC has offered affected workers a course of antibiotics. Pic: CBS

It said the labs had been decontaminated and would reopen "when safe to operate", while workers would be disciplined because proper procedures were not followed.

Tom Skinner, a spokesman for the agency, added: "It's unacceptable and we're going to do everything we can to understand why it happened and what we need to do differently to make sure it doesn't happen again."

A spokesman for the FBI said it was aware of the incident but added there was no evidence of any criminal act.

Scott Becker, of the Association of Public Health Laboratories, said the potential anthrax exposure in Atlanta is the largest since envelopes containing spores were sent through the US postal system in 2001, killing five people.

"It's important to learn what happened so we can ensure it doesn't happen again," he said.

However, labs use anthrax "all the time", he said, adding that the CDC's statement appeared to suggest human error, rather than a "system failure".